The past two hundred years have seen the development of numerous work-producing devices or heat engines. Among these are internal combustion engines such as the diesel engine or cycle, the gasoline engine or Otto cycle and the Wankel rotary engine as well as turbines such as the steam turbine engine or the Rankine cycle and the gas turbine engine or Brayton cycle. The Stirling engine and other cycles have also been defined.
Many work-producing devices or engines utilize a working fluid in the form of a gas. The spark-ignition automotive engine is a familiar example, and the same is true of the diesel engine and the conventional gas turbine. In all of these engines there is a change in composition of the working fluid, because during combustion it changes from air and fuel to combustion products. For this reason these engines are called internal combustion engines. In contrast to this the steam power plant may be called an external-combustion engine, because heat is transferred from the products of combustion to the working fluid. These external-combustion engines or cycles undergo a variety of processes including compression or expansion at varying conditions in order to produce work. The cycles are often defined in terms of these processes. For example, the working fluid in the Rankine cycle ideally undergoes the following steps: a reversible adiabatic pumping process in a pump; a constant-pressure transfer of heat in a boiler; a reversible adiabatic expansion in the turbine or other prime mover such as a steam engine; and a constant-pressure transfer of heat in a condenser.
In the Stirling cycle the heat is transferred to a working fluid during a constant-volume process followed by further heat transfer during an isothermal expansion process. Heat is then rejected during a constant volume process and further during an isothermal compression process.
The most efficient ideal process is the Carnot cycle, which defines the most efficient engine that can be operated between a high temperature and a low temperature reservoir. The Carnot cycle always involves four basic steps, namely: a reversible isothermal process in which heat is transferred to or from the high temperature reservoir; a reversible adiabatic process in which the temperature of the working fluid decreases from the high temperature to the low temperature; a reversible isothermal process in which heat is transferred to or from the low temperature reservoir; and a reversible adiabatic process in which the temperature of the working fluid increases from the low temperature to the high temperature.
In practice all heat engines fall short of ideal performance. This is due to a variety of factors including pressure drops along piping or tubing, heat losses through piping or other surfaces and deviations of the working fluid from the ideal as well as frictional, rotational and other losses, such as due to leakage. Further inefficiencies can result from the configuration of the particular process. These may include one or more of several inefficiencies for a given cycle or engine. For example, many devices fail to develop a sufficient mean effective pressure. Here, the term "mean effective pressure" may be defined as the pressure which, if acted on a piston during the entire power stroke, would do an amount of work equal to that actually done on the piston. The work for one cycle is found by multiplying this mean effective pressure by the area of the piston and by the stroke's length.
In other devices the maximum pressure differential occurs at less than favorable crank angles for exerting forces on the offset of the crank shaft. As such, there is produced a limited amount of energy at the torque producing position(s) of the crank angle. For example, the maximum pressure differential may occur at or near dead center of the piston's travel with concomitant poor crank angle position to produce torque.
Other devices or methods require relatively high operational temperature requirements. Still other methods and devices have limited thermal efficiency in relation to the Carnot cycle. Other devices and methods require relatively high mass flow per unit of power produced, while others suffer from inefficient fuel consumption and incomplete fuel combustion. Other devices and methods have lower efficiencies under partial loads or at lower speeds while others suffer energy losses due to condensation. Still other devices and methods are relatively complex and hence expensive to operate.
These and other shortcomings of the prior devices, including internal and external combustion engines, are alleviated if not eliminated by the present method and apparatus.